Just as you release water vapor when you breathe, plants do, too—although the term "transpire" is more appropriate than "breath." During this process individual water molecules are released from the surface of the plant body through tiny structures called stomata. There are many more individual water vapor molecules inside the air spaces between the tissues of a plant than in the air surrounding the plant body. Consequently water vapor will always exit the plant along a concentration gradient. As more water vapor molecules exit the plant, the remaining water molecules tug on each other and will pull an entire column of water throughout the plant body through special tissues called xylem during the process of transpiration. One way to visualize transpiration is to put a plastic bag around some plant leaves. As Figure 1 shows, transpired water will condense on the inside of the bag. If the bag had been wrapped around the soil below it, too, then even more water vapor would have been released, as water also evaporates from the soil. During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.
Factors affecting transpiration
The amount of water that plants transpire varies greatly geographically and over time. There are a number of factors that determine transpiration rates:
- Temperature: Transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer due to stronger sunlight and warmer air masses. Higher temperatures cause the plant cells which control the openings (stoma), where water is released to the atmosphere, to open, whereas colder temperatures cause the openings to close.
- Relative humidity: As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.
- Wind and air movement: Increased movement of the air around a plant will result in a higher transpiration rate. This is somewhat related to the relative humidity of the air, in that as water transpires from a leaf, the water saturates the air surrounding the leaf. If there is no wind, the air around the leaf may not move very much, raising the humidity of the air around the leaf. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.
- Soil-moisture availability: When soil moisture is lacking, plants can begin to senesce (premature ageing, which can result in leaf loss) and transpire less water.
- Type of plant: Plants transpire water at different rates. Some plants which grow in arid regions—for example, cacti and succulents—conserve precious water by transpiring less water than other plants.
Transpiration and groundwater
In many places, the top layer of the soil where plant roots are located is above the water table and thus is often wet to some extent, but is not totally saturated, as is soil below the water table. The soil above the water table gets wet when it rains as water infiltrates into it from the surface, but it will dry out without additional precipitation. Since the water table is usually below the depth of the plant roots, the plants are dependent on water supplied by precipitation. As Figure 2 shows, in places where the water table is near the land surface, such as next to lakes and oceans, plant roots can penetrate into the saturated zone below the water table, allowing the plants to transpire water directly from the groundwater system. Here, transpiration of groundwater commonly results in a draw down of the water table much like the effect of a pumped well (cone of depression—the dotted line surrounding the plant roots in the diagram).